It fails to be critical thought when you didn't even consider the case of targeting individual pieces of the structure rather than the whole floor or wall, as if a floor or wall is just a single thing and not made of anything else. It's disingenuous. It appears to be thought out but is actually lazy.
I explained why targeting individual pieces of the structure wouldn't work (and would in many cases actually be impossible). Did you read my analysis of why this wouldn't work? If so, do you disagree with it? In what way is it mistaken? Please be specific.
If that's how you'd run it, you should include it as a house rule or have some general rule which touches on the limitations of the "actual" physics you use to mesh mundane and magic in your game.
Except it's not a "house rule". A house rule is when you modify, override, or ignore the existing rules in some way. Please point out which existing rule you think I am modifying, overriding, or ignoring.
Edit: I agree that it's a very good to have a conversation with players to calibrate everyone's expectations appropriately. When I run campaigns, I generally make it clear that as far as I'm concerned, anything the rules don't explicitly call out, can be assumed to work exactly like it does in real life: people have to visit the outhouse (even though there's no pooping in the rules), gravity, magnetism, and other physical forces work exactly like they do IRL (unless some magic is afoot to change that), mundane plants and animals and materials like wood or iron act like they do in reality (except where specified), etc.
At any rate, the player should have some kind of indication before hand that their goal clearly should not work.
Of course. The conversation would go like this:
Player: I cast Shatter on the wall, to make a big hole in it so I can escape!
DM: Um, just so you know, that doesn't actually work. The spell doesn't work that way. (Insert all the explanations I gave in my previous posts, if necessary.) Your character would know that, of course, since he's a wizard and all.
Player: Oh, darn. Hm, ok. In that case, here's what I do instead...
That a concept of load bearing parts of a building even exists and are super commonly understood to be part of any structure, just washing away the entire effort without actually considering how it could be done is not being a good GM and simply shows how little thought you've put into the situation.
Once again, please read my analysis of why "Shatter the load bearing beam" is likely to fail (it's the weight). If you think what I said was mistaken, explain why. That said: the "load bearing part of a building" could apply in some situations. It won't apply to:
1. Break through the wall to escape.
2. Collapse the floor under your enemy (or under yourself, to escape).
It could apply in some small subset of "collapse the ceiling" or "collapse this entire structure" (even less likely). (But, again, probably won't work — as I explained.)
If one of your players chose this spell and, thus, devoted resources to pull off a specific effect, and you just rolled your eyes and said no, you would not be doing your due diligence to enable that player to have fun and participate in the game. ...
This part of your post touches on a broader point, which is why I saved it for last.
No. No, if one of my players chose this spell, didn't read it or didn't understand how it works, and then tried to use it in a situation without thinking about whether it actually makes sense for the spell to apply to that situation — and if I, then, allowed the spell to work — that would mean that I would not be doing my due diligence.
I can't stand playing in games where a GM would let Shatter work as broadly as Aelreth described — never mind that the rules, the actual spell text, forbids it, never minds that it makes no sense if you think about it for more than two seconds — just because it "enables the player to have fun and participate". My interest in playing drops away immediately, because if a GM allows this sort of thing, what that tells me is that either the GM doesn't know the rules (and doesn't care enough to know the rules) — or, even worse, that he just doesn't care, and will let any old thing fly as long as it "enables a player to have fun".
That sort of thing renders choice meaningless. It renders system mastery meaningless. It means that it doesn't matter if I put any effort into picking spells, or thinking of clever ways to use my character's abilities to solve problems — in other words, if I actually exercise creativity within the constraints provided by the rules — because another player, who didn't make any effort, can just say "hey I can use Shatter to Kool-Aid-Man my way through the wall, right?" and the GM will go "why not, lol", because it's "fun" and he's incapable of saying "no". At that point, rules mean nothing, constraints mean nothing, and so creativity and cleverness also mean nothing.
My job as a GM is to know the rules, understand the restrictions, and to be firm, predictable, and consistent in enforcing them; to provide a consistent, coherent world for the players to act within. My job as a player is to know the rules, understand the restrictions, and come up with clever and creative ways to solve the challenges the GM places before me, while operating within those rules and restrictions. That is what I call doing due diligence.